Isolated stack-gate flash cell structure and its contactless flash memory arrays

ABSTRACT

An isolated stack-gate flash cell structure comprises a gate region being formed between common-source/drain regions. The common-source/drain region comprises a pair of divided source/drain diffusion regions being isolated by an etched-back planarized field-oxide layer formed over a shallow trench in a semiconductor substrate and a pair of extended floating-gate spacers being formed over a portion of a pair of etched-back first sidewall dielectric spacers and the etched-back planarized field-oxide layer. The gate region comprises a major floating-gate being formed over a tunneling dielectric layer and integrated with nearby two extended floating-gate spacers to form an integrated floating-gate. A word line over an intergate dielectric layer is formed over the integrated floating-gate. The isolated stack-gate flash cell structure is used to form two contactless parallel divided source/drain diffusion bit-lines arrays.

FIELD OF THE INVENTION

The present invention relates to a stack-gate flash memory cell and its flash memory array and, more particularly, to an isolated stack-gate flash cell structure and its contactless flash memory arrays.

DESCRIPTION OF THE RELATED ART

A stack-gate flash memory cell is known to be a one-transistor cell, in which a gate length of the cell can be defined to be a minimum feature size (F) of technology used. As a consequence, the stack-gate flash memory cell is often used in a high-density flash memory system. The stack-gate flash memory cell can be configured into different array architectures such as NAND, NOR, and AND, based on the basic logic function.

For a NAND-type flash memory array, the stack-gate flash memory cells are connected in series with common-source/drain diffusion regions. In general, a unit cell size of a NAND-type flash memory array is small, but the read speed is slow due to the series resistance of the configuration. For a NOR-type flash memory array, the read speed is much faster, but the unit cell size is larger than that of a NAND-type flash memory array due to the bit-line contacts, and the punch-through effect becomes a major concern for further device scaling if the channel hot-electron injection (CHEI) is used as a programming method. For a AND-type flash memory array, the stack-gate flash memory cells are connected in parallel through the buried source/drain diffusion lines, the unit cell size is slightly larger than that of a NAND-type flash memory array but is smaller than that of a NOR-type flash memory array, and the read speed is faster than that of a NAND-type flash memory array and is slower than that of a NOR-type flash memory array.

FIG. 1A shows a schematic top plan view of a conventional NOR-type flash memory array, in which a cross-sectional view along a A-A′ line is shown in FIG. 1B and a cross-sectional view along a B-B′ line is shown in FIG. 1C. As shown in FIG. 1A, an active region 101 b is defined on a semiconductor substrate 100 by a first masking photoresist step with isolation regions 101 a being formed outside of the active region 101 b; the floating-gate layers 103 together with the control-gate layers 105 are simultaneously patterned by a third masking photoresist step, in which the width of the floating-gate layers 103 is patterned by a second masking photoresist step and the control-gate layer 105 is acted as a word line (WL1 or WL2); the active regions outside of the control-gate layers 105 are implanted with a high dose of doping impurities in a self-aligned manner to form common-source diffusion regions 106 a and common-drain diffusion regions 106 b; an interlayer dielectric layer 107 is formed over a formed structure surface and is then planarized; the contact holes 108 are formed over the common-drain diffusion regions 106 b by a fourth masking photoresist step; a tungsten plug being lined with a barrier metal layer 108 a is formed to fill each of the contact holes 108 and is planarized; and the metal layers 109 being patterned by a fifth masking photoresist step are formed over the tungsten plugs 108 a to act as the bit lines (BL1 and BL2). It is clearly seen that a NOR-type flash memory array shown in FIG. 1A needs at least five masking photoresist steps and a unit cell (UC) size as indicated by a dash square is at least 9F².

FIG. 1B shows a cross-sectional view along a A-A′ line shown in FIG. 1A, in which two stack-gate structures are formed on a semiconductor substrate 100. Each of the stack-gate structures comprises a control-gate layer 105 over an intergate dielectric layer 104 being formed over a floating-gate layer 103; a tunneling dielectric layer 102 being formed under the floating-gate layer 103 and on the semiconductor substrate 100; and a common-source diffusion region 106 a and a common-drain diffusion region 106 b being formed in surface portions of the semiconductor substrate 100 in a self-aligned manner by using the stack-gate structures as implantation masks. A contact hole 108 being filled with a tungsten plug 108 a is formed over a surface portion of the common-drain diffusion region 106 b and a metal layer 109 acting as a bit line is formed over the interlayer dielectric layer 107 and is connected to the tungsten plug 108 a. It is clearly seen from FIG. 1B that the fourth masking photoresist step is required to form the contact hole 108 between two stack-gate structures and misalignment of the contact hole 108 would result in asymmetric parasitic drain resistance for nearby stack-gate structures.

FIG. 1C shows a cross-sectional view along a B-B′ line shown in FIG. 1A, in which a floating-gate layer 103 is formed over a tunneling dielectric layer 102 and two field-oxide (FOX) layers 101 a to increase the coupling ratio; an intergate dielectric layer 104 is formed over the floating-gate layer 103 and portions of the field-oxide layers 101 a; a control-gate layer 105 is formed over the intergate dielectric layer 104; an interlayer dielectric layer 107 is formed over the control-gate layer 107; and a metal layer 109 is formed over a surface portion of the interlayer dielectric layer 107 and is aligned above a middle portion of the tunneling dielectric layer 102. It is clearly seen from FIG. 1C that the second masking photoresist step is required to pattern the width of the floating-gate layer 103 and misalignment between the floating-gate layer 103 with respect to the tunneling dielectric layer 102 in the active region 101 b would occur and will result in asymmetric field distribution near two edges during programming and erasing.

It is, therefore, a major objective of the present invention to offer an isolated stack-gate flash cell structure with an integrated floating-gate being formed along a channel-length direction to largely increase the coupling ratio of a cell.

It is another objective of the present invention to offer a parallel divided source/drain diffusion bit-lines array being fabricated with less masking photoresist steps.

It is a further objective of the present invention to offer a parallel divided source/drain diffusion bit-lines array having a unit cell size of 4F².

Other objectives and advantages of the present invention will be more apparent in a later description.

SUMMARY OF THE INVENTION

The present invention discloses an isolated stack-gate flash cell structure and its contactless flash memory arrays. The isolated stack-gate flash cell structure comprises a gate region being formed between a common-source region and a common-drain region, wherein the gate region being defined by a first masking photoresist step is formed on a semiconductor substrate of a first conductivity type. Each of the common-source/drain regions comprises a pair of divided source/drain diffusion regions of a second conductivity type being formed in side surface portions of the semiconductor substrate, a pair of etched-back first sidewall dielectric spacers over a pair of tunneling dielectric layers being formed on the pair of divided source/drain diffusion regions, an etched-back planarized field-oxide layer being formed over a shallow trench formed between the pair of etched-back first sidewall dielectric spacers, and a pair of extended floating-gate spacers being formed over a portion of the pair of etched-back first sidewall dielectric spacers and the etched-back planarized field-oxide layer in each of the common-source/drain regions. The gate region comprises a major floating-gate being formed over a portion of the tunneling dielectric layer and integrated with nearby two extended floating-gate spacers to form an integrated floating-gate, wherein an implant region of the first conductivity type comprising a shallow implant region for threshold-voltage adjustment and a deep implant region for forming a punch-through stop can be formed in a middle surface portion of the semiconductor substrate in the gate region. A word line together with an intergate dielectric layer is formed over the integrated floating-gate and the etched-back planarized field-oxide layer between the pair of extended floating-gate spacers in each of the common-source/drain regions, wherein the word line, the intergate dielectric layer, and the integrated floating-gate are simultaneously patterned and etched by a second masking photoresist step. A cell isolation region comprising an isolation implant region of the first conductivity type or a shallow trench isolation region being formed in a surface portion of the semiconductor substrate outside of the word line and between the common-source/drain regions.

The contactless flash memory array of the present invention comprises a plurality of gate regions being defined by a first masking photoresist step, wherein each of the plurality of gate regions is formed over a semiconductor substrate of a first conductivity type between common-source/drain regions. Each of the common-source/drain regions comprises a pair of divided source/drain diffusion regions being formed in side surface portions of the semiconductor substrate to act as a pair of buried diffusion bit lines, a pair of etched-back first sidewall dielectric spacers over a pair of tunneling dielectric layers being formed over a pair of divided source/drain diffusion regions, an etched-back planarized field-oxide layer being formed over a shallow trench formed between the pair of etched-back first sidewall dielectric spacers, and a plurality of paired extended floating-gate spacers being formed over the pair of etched-back first sidewall dielectric spacers and a portion of the etched-back planarized field-oxide layer. Each of the plurality of gate regions comprises a plurality of major floating-gates being formed over a portion of the tunneling dielectric layer and integrated with nearby extended floating-gate spacers to form a plurality of integrated floating-gates, wherein an implant region of the first conductivity type comprising a shallow implant region for threshold-voltage adjustment and a deep implant region for forming a punch-through stop can be formed in a middle surface portion of the semiconductor substrate under each of the plurality of major floating-gates. A plurality of word lines together with a plurality of intergate dielectric layers are formed over the plurality of integrated floating-gates and the etched-back planarized field-oxide layers between the plurality of paired extended floating-gate spacers in each of the common-source/drain regions, wherein the plurality of word lines, the plurality of intergate dielectric layers, and the plurality of integrated floating-gates are simultaneously patterned and etched by a second masking photoresist step. A plurality of cell isolation regions comprising a plurality of isolation implant regions or a plurality of shallow trench isolation regions are formed in surface portions of the semiconductor substrate outside of the plurality of word lines and between the common-source/drain regions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A through FIG. 1C show schematic diagrams of a prior art, in which FIG. 1A shows a schematic top plan view of a NOR-type flash memory array; FIG. 1B shows a cross-sectional view along a A-A′ line as indicated in FIG. 1A; and FIG. 1C shows a cross-sectional view along a B-B′ line as indicated in FIG. 1A.

FIG. 2A through FIG. 2F show process steps and their cross-sectional views for forming a first-type isolated stack-gate flash cell structure and its first-type contactless divided source/drain diffusion bit-lines flash memory array of the present invention.

FIG. 3A through FIG. 3E show schematic diagrams of the first-type contactless divided source/drain diffusion bit-lines flash memory array of the present invention, in which FIG. 3A shows a schematic top plan view; FIG. 3B shows a cross-sectional view along a B-B′ line as indicated in FIG. 3A; FIG. 3C shows a cross-sectional view along a C-C′ line as indicated in FIG. 3A; FIG. 3D shows a cross-sectional view along a D-D′ line as indicated in FIG. 3A; and FIG. 3E shows a schematic circuit representation.

FIG. 4A through FIG. 4F show simplified process steps after FIG. 2C and their cross-sectional views for forming a second-type isolated stack-gate flash cell structure and its second-type contactless divided source/drain diffusion bit-lines flash memory array of the present invention.

FIG. 5A through FIG. 5E show schematic diagrams of the second-type contactless divided source/drain diffusion bit-lines flash memory array of the present invention, in which FIG. 5A shows a schematic top plan view; FIG. 5B shows a cross-sectional view along a B-B′ line as indicated in FIG. 5A; FIG. 5C shows a cross-sectional view along a C-C′ line as indicated in FIG. 5A; FIG. 5D shows a cross-sectional view along a D-D′ line as indicated in FIG. 5A; and FIG. 5E shows a schematic circuit representation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 2A through FIG. 2F, there are shown process steps and their cross-sectional views for forming a first-type isolated stack-gate flash cell structure and its first-type contactless divided source/drain diffusion bit-lines flash memory array of the present invention.

FIG. 2A shows that a tunneling dielectric layer 201 is formed on a semiconductor substrate 200 of a first conductivity type; a first conductive layer 202 is then formed over the tunneling dielectric layer 201; and a masking dielectric layer 203 is formed over the first conductive layer 202. The tunneling dielectric layer 201 is preferably a thermal oxide layer or a nitrided thermal oxide layer and its thickness is preferably between 70 Angstroms and 120 Angstroms. The first conductive layer 202 is preferably made of doped polycrystalline silicon or doped amorphous silicon as deposited by low-pressure chemical vapor deposition (LPCVD) technique and its thickness is preferably between 1000 Angstroms and 3000 Angstroms. The masking dielectric layer 203 is preferably made of silicon nitride as deposited by LPCVD and its thickness is preferably between 1000 Angstroms and 3000 Angstroms.

FIG. 2B shows that a structure shown in FIG. 2A is patterned by a first masking photoresist (PR1) step (not shown) to define a plurality of gate regions (GR); the masking dielectric layer 203 and the first conductive layer 202 in each of common-source/drain regions (CS/DR) are sequentially removed by anisotropic dry etching; and subsequently, common-source/drain diffusion regions 204 a are formed by implanting doping impurities across the tunneling dielectric layer 201 into the semiconductor substrate 200 in a self-aligned manner. There are several doping structures for the common-source/drain diffusion regions 204 a: the common-source/drain diffusion region 204 a can be a heavily-doped diffusion region of a second conductivity type or a shallow heavily-doped diffusion region of a second conductivity type being formed within a lightly-doped diffusion region of the second conductivity type; the common-source diffusion region can be a shallow heavily-doped diffusion region of the second conductivity type being formed within a lightly-doped diffusion region of the second conductivity type and the common-drain diffusion region can be a shallow heavily-doped diffusion region of the second conductivity type or a shallow heavily-doped diffusion region of the second conductivity type being formed within a moderately-doped diffusion region of the first conductivity type. It should be emphasized that different doping types or structures in the common-source diffusion regions and the common—drain diffusion regions need at least an additional masking photoresist step.

FIG. 2C shows that a pair of first sidewall dielectric spacers 205 a are formed over sidewalls of nearby gate regions (GR) and on a portion of the tunneling dielectric layer 201 in each of the common-source/drain regions (CS/DR); the tunneling dielectric layers 201 between the pair of first sidewall dielectric spacers 205 a is removed and the semiconductor substrate 200 is then etched to form a shallow trench in each of the common-source/drain regions (CS/DR); a planarized field-oxide (FOX) layer 206 a is formed to fill a gap in each of the common-source/drain regions (CS/DR). The first sidewall dielectric spacer 205 a is preferably made of silicon dioxide as deposited by LPCVD and is formed by first depositing a silicon dioxide layer 205 and then etching back a thickness of the deposited silicon dioxide layer 205. The planarized field-oxide layer 206 a is preferably deposited by LPCVD, high-density plasma (HDP) CVD, or plasma-enhanced (PE) CVD and is formed by first depositing a thick silicon dioxide layer 206 to fill a gap in each of the common-source/drain regions (CS/DR) and then planarizing the deposited thick silicon dioxide layer 206 by using chemical-mechanical polishing (CMP) with the patterned masking dielectric layer 203 a as a polishing stop. It should be noted that the tunneling dielectric layer 201 in each of the common-source/drain regions (CS/DR) can be removed before forming the pair of first sidewall dielectric spacers 205 a.

FIG. 2D shows that the pair of first sidewall dielectric spacers 205 a and the planarized field-oxide layers 206 a in each of the common-source/drain regions (CS/DR) are etched back to have a thickness between 500 Angstroms and 1000 angstroms to form etched-back first sidewall dielectric spacers 205 b and etched-back planarized field-oxide layers 206 b.

FIG. 2E shows that the patterned masking dielectric layers 203 a are selectively removed by using hot phosphoric acid or anisotropic dry etching; and a pair of sidewall conductive spacers 207 a are formed over sidewalls of nearby patterned first conductive layers 202 a in each of the common-source/drain regions (CS/DR). The pair of sidewall conductive spacers 207 a are preferably made of doped polycrystalline silicon or doped amorphous silicon as deposited by LPCVD and are formed by first depositing a second conductive layer 207 and then etching back a thickness of the deposited second conductive layer 207. It should be noted that the spacer width of the pair of sidewall conductive spacers 207 a is mainly controlled by a thickness of the deposited second conductive layer 207.

FIG. 2F shows that an intergate dielectric layer 208 is formed over a structure surface shown in FIG. 2E; a third conductive layer 209 is then formed over the intergate dielectric layer 208; a metal layer 210 is subsequently formed over the third conductive layer 209; and subsequently, the metal layer 210, the third conductive layer 209, the intergate dielectric layer 208, the pair of sidewall conductive spacers 207 a, and the patterned first conductive layers 202 a are simultaneously patterned by a second masking photoresist (PR2) step (not shown) to form a plurality of word lines 210 a/209 a over a plurality of intergate dielectric layers 208 a being formed over a plurality of integrated floating-gates 202 b/207 b, wherein the plurality of word lines 210 a/209 a are formed transversely to the divided source/drain diffusion regions 204 b. It should be noted that a plurality of cell isolation regions 211 a (not shown) are formed in surface portions of the semiconductor substrate 200 outside of the plurality of word lines 210 a/209 a and between nearby common-source/drain regions (CS/DR). Each of the plurality of cell isolation regions 211 a comprises an isolation implant region of the first conductive type or a shallow trench isolation (STI) region. The intergate dielectric layer 208 is preferably an oxide-nitride-oxide (ONO) layer and its equivalent oxide thickness is preferably between 80 Angstroms and 150 Angstroms. The third conductive layer 209 is preferably made of doped polycrystalline silicon or doped amorphous silicon as deposited by LPCVD and its thickness is preferably between 1500 Angstroms and 2500 Angstroms. The metal layer 210 is preferably a copper (Cu) or aluminum (Al) layer being formed over a barrier metal layer such as a titanium nitride (TiN) or tantalum nitride (TaN) layer. It should be noted that the metal layer 210 can be a tungsten (W) layer over a barrier metal layer or a tungsten disilicide (WSi₂) layer. It is clearly seen from FIG. 2F that the integrated floating-gate 202 b/207 b comprising a major floating-gate 202 b and two extended floating-gate spacers 207 b may largely increase the coupling ratio of an isolated stack-gate flash cell as indicated by a dash square and two masking photoresist steps are required to form a first-type contactless parallel divided source/drain diffusion bit-lines array of the present invention.

Referring now to FIG. 3A, there is shown a schematic top plan view of the first type isolated stack-gate flash cell structure and its first-type contactless parallel divided source/drain diffusion bit-lines array of the present invention, in which a cross-sectional view along a A-A′ line is shown in FIG. 2F. As shown in FIG. 3A, a plurality of gate regions (GR) are defined alternately on the semiconductor substrate 200, wherein each of the plurality of gate regions (GR) is formed between common-source/drain regions (CS/DR); a pair of divided source/drain diffusion regions 204 b being formed in each of the common-source/drain regions (CS/DR) are acted as a pair of buried diffusion bit lines (BL1 and BL2); a shallow trench isolation (STI) region being filled with an etched-back planarized field-oxide layer 206 b is formed between a pair of etched-back first sidewall dielectric spacers 205 b in each of the common-source/drain regions (CS/DR); a plurality of word lines (WL's) 210 a/209 a are defined to be perpendicular to the buried diffusion bit lines (BL1's and BL2's) and are formed above the integrated floating-gates 202 b/207 b; and a plurality of cell isolation regions 211 a are formed in surface portions of the semiconductor substrate 200 outside of the plurality of word lines (WL's) 210 a/209 a and between nearby common-source/drain regions (CS/DR). A unit cell (UC) size as indicated by a dash square can be made to be equal to 4F² if the width of the cell isolation region 211 a and the width of the word line 210 a/209 a are defined to be a minimum feature size (F).

FIG. 3B shows a cross-sectional view along a B-B′ line as indicated in FIG. 3A, wherein an etched-back planarized field-oxide layer 206 b is formed over a STI region in each of the common-source/drain regions (CS/DR); a plurality of word lines 210 a/209 a over a plurality of intergate dielectric layers 208 a are simultaneously patterned and etched by the second masking photoresist step.

FIG. 3C shows a cross-sectional view along a C-C′ line as indicated in FIG. 3A, wherein the plurality of word lines 210 a/209 a, the plurality of intergate dielectric layers 208 a, and the plurality of extended floating-gate spacers 207 b are simultaneously patterned and etched by the second masking photoresist step; and the etched-back first sidewall dielectric spacer 205 b over the tunneling dielectric layer 201 a are formed over each of the divided source/drain diffusion regions 204 b.

FIG. 3D shows a cross-sectional view along a D-D′ line as indicated in FIG. 3A, wherein the plurality of word lines 210 a/209 a, the plurality of intergate dielectric layers 208 a, and the plurality of major floating-gates 202 b are simultaneously patterned and etched by the second masking photoresist step and an isolation implant region 211 a is formed in a surface portion of the semiconductor substrate 200 between nearby word lines 210 a/209 a. It should be noted that the tunneling dielectric layer 201 a between nearby word lines 210 a/209 a can be removed and the semiconductor substrate 200 between nearby word lines 210 a/209 a can be anisotropically etched to form a shallow trench.

FIG. 3E shows a schematic circuit representation of the first-type contactless parallel divided source/drain bit-lines array, in which a plurality of first-type isolated stack-gate flash cell structures are formed between the divided source/drain diffusion regions 204 b (BL1 and BL2) being acted as a plurality of buried diffusion bit-lines (BL1's and BL2's) and the plurality of word lines (WL's) 210 a/209 a are formed transversely to the plurality of buried diffusion bit lines (BL1's and BL2's).

Referring now to FIG. 4A through FIG. 4F, there are shown simplified process steps after FIG. 2C and their cross-sectional views for forming a second-type isolated stack-gate flash cell structure and its second-type contactless parallel divided source/drain diffusion bit-lines flash memory array.

FIG. 4A shows that the patterned masking dielectric layers 203 a shown in FIG. 2C are selectively removed by hot phosphoric acid or anisotropic dry etching; a pair of second sidewall dielectric spacers 212 a are then formed over sidewalls of nearby first sidewall dielectric spacers 205 a and on a portion of the patterned first conductive layer 202 a in each of the plurality of gate regions (GR). The pair of second sidewall dielectric spacers 212 a are preferably made of silicon nitride as deposited by LPCVD and are formed by first depositing a silicon nitride layer 212 and then etching back a thickness of the deposited silicon nitride layer 212.

FIG. 4B shows that the patterned first conductive layer 202 a between the pair of second sidewall dielectric spacers 212 a is etched to form a shallow trench in each of the patterned first conductive layers 202 a; and an ion-implantation is performed in a self-aligned manner to form an implant region 213 a of the first conductivity type in a surface portion of the semiconductor substrate 200 between the pair of second sidewall dielectric spacers 212 a. The implant region 213 a comprises a shallow implant region as indicated by a dash line for threshold-voltage adjustment and a deep implant region as indicated by cross symbols for forming a punch-through stop.

FIG. 4C shows that a planarized conductive layer 214 a (not shown) is formed over a gap between the pair of second sidewall dielectric spacers 212 a in each of the plurality of gate regions (GR); and the planarized conductive layer 214 a is etched back to a depth equal to a top surface level of the patterned first conductive layer 202 b to form an etched-back planarized conductive layer 202 d. The planarized conductive layer 214 a is preferably made of doped polycrystalline silicon or doped amorphous silicon as deposited by LPCVD and is formed by first depositing a conductive layer 214 to fill a gap between the pair of second sidewall dielectric spacers 212 a and then planarizing the deposited conductive layer 214 using CMP with the pair of second sidewall dielectric spacers 212 a as a polishing stop.

FIG. 4D shows that the pair of first sidewall dielectric spacers 205 a and the planarized field-oxide layer 206 a in each of the common-source/drain regions (CS/DR) are etched back to have a thickness between 500 Angstroms and 1000 Angstroms; the pair of second sidewall dielectric spacers 212 a are then removed by hot phosphoric acid or anisotropic dry etching; and subsequently, a pair of sidewall conductive spacers 207 a are formed over sidewalls of nearby patterned first conductive layers 202 b and on the pair of etched-back first sidewall dielectric spacers 205 b and a portion of the etched-back planarized field-oxide layer 206 b in each of the common-source/drain regions (CS/DR). The pair of sidewall conductive spacers 207 a are preferably made of doped polycrystalline silicon or doped amorphous silicon as deposited by LPCVD and are formed by first depositing a second conductive layer 207 and then etching back a thickness of the deposited second conductive layer 207.

FIG. 4E shows that an intergate dielectric layer 208 is formed over a structure surface shown in FIG. 4D; a third conductive layer 209 is then formed over the intergate dielectric layer 208; and a metal layer 210 is formed over the third conductive layer 209.

FIG. 4F shows that the metal layer 210, the third conductive layer 209, the pair of sidewall conductive spacers 207 a, and the patterned first conductive layers 202 b together with the etched-back planarized conductive layers 202 d are simultaneously patterned and etched by a second masking photoresist (PR2) step (not shown) to form a plurality of word lines (WL's) 210 a/209 a as described in FIG. 2F. Similarly, a plurality of cell isolation regions 211 a (not shown) are formed in surface portions of the semiconductor substrate 200 outside of the plurality of word lines (WL's) 210 a/209 a and between the common-source/drain regions (CS/DR). A unit cell (UC) size as marked by a dash square can be made to be equal to 4F² if the width of the word line 210 a/209 a and the width of the cell isolation region 211 a are defined to be a minimum feature size (F) of technology used. It is clearly seen that FIG. 4F is the same as FIG. 2F except that an implant region 213 a is formed in a middle surface portion of the semiconductor substrate 200 under the major floating-gate 202 c/202 e to improve the punch-through effect of a scaled cell.

Referring now to FIG. 5A, there is shown a schematic top plan view of the second-type contactless parallel divided source/drain diffusion bit-lines array of the present invention, in which a cross-sectional view along a A-A′ line as indicated in FIG. 5A is shown in FIG. 4F. Comparing FIG. 5A to FIG. 3A, it is clearly seen that FIG. 5A is the same as FIG. 3A except that an implant region 213 a is formed under each of the etched-back planarized conductive islands 202 e as marked in FIG. 5A. Therefore, a further description for FIG. 5A can refer to FIG. 3A.

FIG. 5B shows a cross-sectional view along a B-B′ line as indicated in FIG. 5A, which is the same as FIG. 3B. A further description of FIG. 5B can refer to that of FIG. 3B.

FIG. 5C shows a cross-sectional view along a C-C′ line as indicated in FIG. 5A, which is also the same as FIG. 3C. Similarly, a further description of FIG. 5C can refer to that of FIG. 3C.

FIG. 5D shows a cross-sectional view along a D-D′ line as indicated in FIG. 5A, in which a major floating-gate 202 b in FIG. 3D is replaced by an etched-back planarized conductive island 202 e being formed within the patterned first conductive island 202 c in FIG. 5D and an implant region 213 a as described is formed in a surface portion of the semiconductor substrate 200 under the etched-back planarized conductive island 202 e. A further description of FIG. 5D can refer to that of FIG. 3D.

FIG. 5E shows a schematic circuit representation of the second-type contactless parallel divided source/drain diffusion bit-lines array of the present invention. It is clearly seen that FIG. 5E is the same as FIG. 3E except that a circle is marked under a middle portion of the major floating-gate 202 e/202 c for each of the second-type isolated stack-gate flash cell structure to indicate an implant region 213 a being formed to improve the punch-through effect of the scaled cell.

Accordingly, the features and advantages of the present invention as described above can be summarized as follows:

(a) The isolated stack-gate flash cell structure of the present invention offers an integrated floating-gate to largely increase the coupling ratio without an additional masking photoresist step.

(b) The isolated stack-gate flash cell structure of the present invention can offer a unit cell size of 4F².

(c) The isolated stack-gate flash cell structure of the present invention can offer an implant region in a middle portion of the semiconductor substrate under the major floating-gate to improve the punch-through effect of the scaled cell.

(d) The isolated stack-gate flash cell structure and its contactless parallel isolated source/drain diffusion bit-lines array of the present invention can be fabricated by using only two masking photoresist steps.

(e) The contactless parallel divided source/drain diffusion bit-lines array of the present invention offers a plurality of metal word lines to largely reduce the word-line resistance of a high-density flash memory array.

(f) The contactless parallel divided source/drain diffusion bit-lines array of the present invention offers higher flexibility for programming and erasing.

While the present invention has been particularly shown and described with a reference to the present examples and embodiments as considered as illustrative and not restrictive. Moreover, the present invention is not to be limited to the details given herein, it will be understood by those skilled in the art that various changes in form and details may be made without departure from the true spirit and scope of the present invention. 

What is claimed is:
 1. A stack-gate flash cell structure, comprising: a semiconductor substrate of a first conductivity type; a cell region comprising a gate region being formed over said semiconductor substrate, wherein said gate region being formed between a common-source region and a common-drain region is defined by a first masking photoresist step; said common-source region comprising a pair of divided source diffusion regions being formed in first side surface portions of said semiconductor substrate, a pair of etched-back first sidewall dielectric spacers over first portions of a tunneling dielectric layer being formed over said pair of divided source diffusion regions, an etched-back planarized field-oxide layer being formed over a shallow trench between said pair of etched-back first sidewall dielectric spacers, and a pair of extended floating-gate spacers being formed on a portion of said pair of etched-back first sidewall dielectric spacers and said etched-back planarized field-oxide layer; said common-drain region comprising a pair of divided drain diffusion regions being formed in second side surface portions of said semiconductor substrate, said pair of etched-back first sidewall dielectric spacers over second portions of said tunneling dielectric layer being formed over said pair of divided drain diffusion regions, said etched-back planarized field-oxide layer being formed over said shallow trench between said pair of etched-back first sidewall dielectric spacers, and said pair of extended floating-gate spacers being formed on a portion of said pair of etched-back first sidewall dielectric spacers and said etched-back planarized field-oxide layer; said gate region comprising a major floating-gate being formed over a third portion of said tunneling dielectric layer, wherein said major floating-gate is integrated with nearby two extended floating-gate spacers to form an integrated floating-gate; a word line over an intergate dielectric layer being formed over said integrated floating-gate and said etched-back planarized field-oxide layers outside of said integrated floating-gate, wherein said word line, said intergate dielectric layer, and said integrated floating-gate are simultaneously patterned and etched by using a second masking photoresist step; and a cell isolation region being formed in each side surface portion of said semiconductor substrate outside of said word line and between said common-source region and said common-drain region.
 2. The stack-gate flash cell structure according to claim 1, wherein said divided source diffusion region comprises a heavily-doped source diffusion region of a second conductivity type or a shallow heavily-doped source diffusion region of a second conductivity type being formed within a lightly-doped source diffusion region of said second conductivity type.
 3. The stack-gate flash cell structure according to claim 1, wherein said divided drain diffusion region comprises a shallow heavily-doped drain diffusion region of a second conductivity type being formed within a lightly-doped source diffusion region of said second conductivity type or a heavily-doped drain diffusion region of a second conductivity type.
 4. The stack-gate flash cell structure according to claim 1, wherein said divided drain diffusion region comprises a shallow heavily-doped drain diffusion region of a second conductivity type being formed within a moderately-doped drain diffusion region of said first conductivity type.
 5. The stack-gate flash cell structure according to claim 1, wherein an implant region of said first conductivity type comprising a shallow implant region for threshold-voltage adjustment and a deep implant region for forming a punch-through stop is formed in a middle surface portion of said semiconductor substrate under said major floating-gate and is located between a pair of second sidewall dielectric spacers being formed over sidewalls of said gate region and on a portion of said major floating-gate.
 6. The stack-gate flash cell structure according to claim 1, wherein said cell isolation region comprises an isolation implant region of said first conductivity type or a shallow trench isolation region.
 7. The stack-gate flash cell structure according to claim 1, wherein said word line comprises a metal layer over a barrier metal layer being formed over a doped polycrystalline silicon or doped amorphous silicon layer or a tungsten disilicide (WSi₂) layer being formed over a doped polycrystalline silicon or doped amorphous silicon layer.
 8. A stack-gate flash cell structure, comprising: a semiconductor substrate of a first conductivity type; a cell region comprising a gate region being formed over said semiconductor substrate, wherein said gate region being formed between a common-source region and a common-drain region is defined by a first masking photoresist step; said common-source region comprising a pair of divided source diffusion regions of a second conductivity type being formed in first side surface portions of said semiconductor substrate, a pair of etched-back first sidewall dielectric spacers over first portions of a tunneling dielectric layer being formed over said pair of divided source diffusion regions, an etched-back planarized field-oxide layer being formed over a shallow trench between said pair of etched-back first sidewall dielectric spacers, and a pair of extended floating-gate spacers being formed on a portion of said pair of etched-back first sidewall dielectric spacers and said etched-back planarized field-oxide layer; said common-drain region comprising a pair of divided drain diffusion regions being formed in second side surface portions of said semiconductor substrate, said pair of etched-back first sidewall dielectric spacers over second portions of said tunneling dielectric layer being formed over said pair of divided drain diffusion regions, said etched-back planarized field-oxide layer being formed over said shallow trench between said pair of etched-back first sidewall dielectric spacers, and said pair of extended floating-gate spacers being formed on a portion of said pair of etched-back first sidewall dielectric spacers and said etched-back planarized field-oxide layer; said gate region comprising a major floating-gate being formed over a third portion of said tunneling dielectric layer, wherein said major floating-gate is integrated with nearby two extended floating-gate spacers to form an integrated floating-gate; a word line over an intergate dielectric layer being formed over said integrated floating-gate and said etched-back planarized field-oxide layers outside of said integrated floating-gate, wherein said word line, said intergate dielectric layer, and said integrated floating-gate are simultaneously patterned and etched by using a second masking photoresist step; and a cell isolation region being formed in each side surface portion of said semiconductor substrate outside of said word line and between said common-source region and said common-drain region.
 9. The stack-gate flash cell structure according to claim 8, wherein said divided drain diffusion region comprises a heavily-doped drain diffusion region of said second conductivity type or a shallow heavily-doped drain diffusion region of said second conductivity type being formed within a lightly-doped drain diffusion region of said second conductivity type.
 10. The stack-gate flash cell structure according to claim 8, wherein said divided drain diffusion region comprises a heavily-doped drain diffusion region of said second conductivity type being formed within a moderately-doped drain diffusion region of said first conductivity type.
 11. The stack-gate flash cell structure according to claim 8, wherein said word line comprises a metal layer over a barrier metal layer being formed over a doped polycrystalline silicon or doped amorphous silicon layer or a tungsten disilicide (WSi₂) layer being formed over a doped polycrystalline silicon or doped amorphous silicon layer.
 12. The stack-gate flash cell structure according to claim 8, wherein an implant region of said first conductivity type comprising a shallow implant region for threshold-voltage adjustment and a deep implant region for forming a punch-through stop is formed in a middle surface portion of said semiconductor substrate under said major floating-gate and is located between a pair of second sidewall dielectric spacers being formed over sidewalls of said gate region and on a portion of said major floating-gate.
 13. A contactless flash memory array, comprising: a semiconductor substrate of a first conductivity type; a plurality of gate regions being alternately formed over said semiconductor substrate, wherein each of the plurality of gate regions being formed between a common-source region and a common-drain region is defined by a first masking photoresist step; said common-source region comprising a pair of divided source diffusion regions being formed in first side surface portions of said semiconductor substrate to act as a portion of buried source diffusion bit lines, a pair of etched-back first sidewall dielectric spacers over first portions of a tunneling dielectric layer being formed over said pair of divided source diffusion regions, an etched-back planarized field-oxide layer being formed over a shallow trench between said pair of etched-back first sidewall dielectric spacers, and a plurality of paired extended floating-gate spacers being formed on a portion of said pair of etched-back first sidewall dielectric spacers and said etched-back planarized field-oxide layer; said common-drain region comprising a pair of divided drain diffusion regions being formed in second side surface portions of said semiconductor substrate to act as a portion of buried drain diffusion bit lines, said pair of etched-back first sidewall dielectric spacers over second portions of said tunneling dielectric layer being formed over said pair of divided drain diffusion regions, said etched-back planarized field-oxide layer being formed over said shallow trench between said pair of etched-back first sidewall dielectric spacers, and the plurality of paired extended floating-gate spacers being formed on a portion of said pair of etched-back first sidewall dielectric spacers and said etched-back planarized field-oxide layer; each of the plurality of gate regions comprising a plurality of major floating-gates being formed over a third portion of said tunneling dielectric layer, wherein each of the plurality of major floating-gates is integrated with nearby two extended floating-gate spacers to form an integrated floating-gate; a plurality of word lines over a plurality of intergate dielectric layers being formed over a surface formed alternately by said integrated floating-gate and said etched-back planarized field-oxide layer between nearby integrated floating-gates, wherein the plurality of word lines, the plurality of intergate dielectric layers, and said integrated floating-gates are simultaneously patterned and etched by using a second masking photoresist step; and a plurality of cell isolation regions being formed in surface portions of said semiconductor substrate outside of the plurality of word lines and between said common-source region and said common-drain region.
 14. The contactless flash memory array according to claim 13, wherein said divided source diffusion region comprises a heavily-doped source diffusion region of a second conductivity type or a shallow heavily-doped source diffusion region of a second conductivity type being formed within a lightly-doped source diffusion region of said second conductivity type.
 15. The contactless flash memory array according to claim 13, wherein said divided drain diffusion region comprises a heavily-doped drain diffusion region of a second conductivity type or a shallow heavily-doped drain diffusion region of a second conductivity type being formed within a lightly-doped drain diffusion region of said second conductivity type.
 16. The contactless flash memory array according to claim 13, wherein said divided drain diffusion region comprises a shallow heavily-doped drain diffusion region of a second conductivity type being formed within a moderately-doped drain diffusion region of said first conductivity type.
 17. The contactless flash memory array according to claim 13, wherein each of the plurality of cell isolation regions comprises an isolation implant region of said first conductivity type or a shallow trench isolation region.
 18. The contactless flash memory array according to claim 13, wherein each of the plurality of word lines comprises a metal layer over a barrier metal layer being formed over a doped polycrystalline silicon or doped amorphous silicon layer or a tungsten disilicide (WSi₂) layer being formed over a doped polycrystalline silicon or doped amorphous silicon layer.
 19. The contactless flash memory array according to claim 13, wherein an implant region of said first conductivity type comprising a shallow implant region for threshold-voltage adjustment and a deep implant region for forming a punch-through stop is formed in a middle surface portion of said semiconductor substrate under each of the plurality of major floating-gates.
 20. The contactless flash memory array according to claim 13, wherein each of the plurality of intergate dielectric layers comprises an oxide-nitride-oxide (ONO) layer and said tunneling dielectric layer comprises a thermal oxide layer or a nitrided thermal oxide layer. 